The present invention relates to the field of the enhanced recovery of crude oil from underground formations, and more particularly to the problems of retention of surfactants in these underground formations during steps of enhanced oil recovery.
During the extraction of oil from a hydrocarbon reservoir (oil-yielding reservoir such as a consolidated or non-consolidated rock formation, or a sand, for example), according to a first step known as “primary recovery”, the oil is entrained out of a production well by the excess pressure naturally prevailing in the reservoir. This primary recovery gives access to only a small amount of the oil contained in the reservoir, typically at the very most about 10% to 15%.
In order to enable the extraction of the oil to continue after this primary recovery, secondary production methods are employed, when the pressure in the reservoir becomes insufficient to displace the oil that is still in place. Typically, a fluid is injected (reinjection of the diluted or undiluted water produced, injection of sea or river water, or alternatively injection of gas, for example) into the hydrocarbon reservoir, in order to exert in the reservoir an excess pressure capable of entraining the oil toward the production well(s). A usual technique in this context is injection of water (also known as inundation or waterflooding), in which large volumes of water are injected under pressure into the reservoir via injection wells. The injected water entrains part of the oil that it encounters and pushes it toward one or more production wells. However, secondary production methods such as waterflooding make it possible to extract only a relatively small part of the hydrocarbons in place (typically about 30%). This partial flushing is due especially to the trapping of the oil by the capillary forces, to the differences in viscosity and density existing between the injected fluid and the hydrocarbons in place, and also to heterogeneities at microscopic or macroscopic scales (at the scale of the pores and also at the scale of the reservoir).
To attempt to recover the rest of the oil which remains in the underground formations after the use of the primary and secondary production methods, various techniques have been proposed, which are known as Enhanced Oil Recovery. Among these techniques, mention may be made of techniques similar to the abovementioned waterflooding (inundation), but using a water comprising additives, for instance water-soluble surfactants (this is then typically referred to as surfactant flooding). The use of such surfactants especially induces a decrease in the water/oil interface tension, which is capable of ensuring more efficient entrainment of the oil trapped in the pore constrictions.
The surfactants usually recommended in this context are typically anionic surfactants, especially of sulfate or sulfonate type. Although they do indeed prove to be effective in lowering the water/oil interface tension, these anionic surfactants have a drawback, which is that they tend to remain trapped in the underground formations, typically taking into account chemical absorption phenomena at the surface of the rocks, which appreciably affects the recovery efficacy and/or the process costs.
Among other things, the surfactants thus immobilized in the reservoir can no longer participate in the mobilization and extraction of the oil, and the extraction efficacy is consequently affected. Strong adsorption may be compensated for by the use of high concentrations of surfactants, but with repercussions in terms of costs. More generally, the surfactant adsorption phenomena have a negative impact on the extraction costs.
The adsorption phenomena of the abovementioned anionic surfactants are most particularly pronounced:                when the surfactants are used in a water with high contents of salt and/or of divalent cations (especially seawater); and        in certain rock formations, such as carbonates or clay rocks such as argillaceous sandstones (in which the adsorption is high, even if waters with high contents of salt and/or of divalent ions are avoided).        
To inhibit the surfactant adsorption phenomena, various solutions have been proposed, which are more or less effective and which are generally limited to specific conditions of use.
Thus, in particular, it has been proposed to use sacrificial agents, which are supposed to have greater affinity for the rock than the surfactant species employed for the enhanced oil recovery. In this context, the possible use of lignosulfonates or of poly(ethylene oxides) of low molecular mass, whose efficacy is variable, especially depending on the nature of the surfactants, of the rock and of the salinity conditions, has been disclosed.
More recently, the use of ethoxylated nonionic surfactants, such as Rhodasurf® LA 12 available from the company Solvay, which make it possible to effectively overcome the harmful effects of the retention of anionic surfactants in oil reservoirs, has been proposed, especially in application FR 2 986 008.